Ceramic enamel paint compositions are well known in the art. They can be used for a variety of applications as, for example, decorative coatings for glassware, chinaware, and the like. They are especially useful in forming colored borders around glass sheets used for automotive windshields, sidelights and backlights. The colored borders enhance appearance as well as prevent UV degradation of underlying adhesives.
In general, these enamel compositions consist mainly of a glass frit, a colorant and an organic vehicle. They are applied to a substrate and subsequently fired to burn off the organic vehicle and fuse the frit thus bonding the enamel coating to the substrate.
Glass sheets for automotive use are generally coated with the ceramic enamel composition and then subjected to a forming process at elevated temperatures. During this treatment the enamel melts and fuses to the glass substrate and the glass is formed into a desired final shape. However, many prior art coatings exhibit a tendency to adhere to the materials covering the forming die, such as a fiberglass or metal fiber covered die, because these conventional enamels have a low viscosity after melting and stick to other materials at high temperature. Accordingly, these materials are not suitable for use in glass forming processes in which the heated glass coated with enamel contacts a material-covered forming die.
Various approaches have been suggested in order to facilitate the forming of glass sheets with a ceramic enamel coated thereon, withstanding of the elevated bending or forming temperatures, and repeated contact of the glass sheet and the covered forming die without the enamel adhering to the forming die. For example, U.S. Pat. Nos. 4,596,590 and 4,770,685 (issued to Boaz) and EP 201241 propose the addition of a low valent metal oxide powder, e.g., cuprous oxide, to the paint composition to provide a non-stick barrier between the coating and the fiberglass-covered forming die. U.S. Pat. Nos. 4,684,389; 4,857,096 and 5,037,783 (issued to Boaz), propose adding finely divided zinc metal powder for a similar effect. The use of an iron metal powder is proposed in U.S. Pat. No. 4,983,196 (issued to Stotka).
The use of other metal oxide powders, including bismuth oxide-containing formulations have been proposed in U.S. Pat. No. U.S. Pat. No. 4,029,605 (issued to Kosiorek); U.S. Pat. No. 4,098,949 (issued to Kosiorek); U.S. Pat. No. 4,892,847 (issued to Reinherz); U.S. Pat. No. 4,959,090 (issued to Reinherz); U.S. Pat. No. 4,970,178 (issued to Klimas et al.); U.S. Pat. No. 5,093,285 (issued to Murkens); U.S. Pat. No. 5,203,902 (issued to Murkens) and EP 370,693.
However, certain of the bismuth containing systems produce a weak glass, such as when lithium is present, and have poor silver bleed-through properties, as well as inadequate anti-stick properties.
U.S. Pat. No. 4,828,596 proposes introducing copper sulfide or copper sulfate into the ceramic enamel composition as an anti-stick agent. Sulfides or sulfates of other metals have been proposed as anti-stick agents in U.S. Pat. No. 4,822,396 (issued to Reinherz et al.).
A purportedly improved anti-stick ceramic enamel composition is proposed by U.S. Pat. Nos. 5,153,150 and 5,208,191 (issued to Ruderer et al.) wherein a seed powder containing Zn.sub.2 SiO.sub.4 is introduced into an oxide composition with or without the presence of bismuth oxide.
As a further problem, a number of previous ceramic enamel systems employ a lead-containing glass frit. For environmental considerations it is desirable to avoid the use of any lead-containing system.
Also, while some of the above-mentioned enamel systems may perform fairly well in conventional glass forming processes, some are not satisfactory for use in the newly-developed "deep bend" processes for forming automotive glass.
It is essential that the enamel compositions also possess adequate resistance to certain chemical agents, which they may contact, and many of the prior art compositions fail in this respect.
Each of the previously known enamel compositions suffers from one or more of the deficiencies noted above, making it desirable to provide a composition which avoids these shortcomings. The present invention provides such ceramic enamel compositions, a method for employing the compositions, and substrates coated therewith.